Sipes are commonly provided in a tread surface to enhance water absorption in order to enhance traveling performance on wet road surfaces and icy and snowy road surfaces. However, if an excessive number of sipes are disposed in the tread surface, tread rigidity will decline, leading to steering stability and braking ability being negatively affected. Therefore, conventionally, various technologies have been proposed regarding the form and arrangement of the sipe (e.g. see Japanese Unexamined Patent Application No. H9-263111A and Japanese Unexamined Patent Application Publication No. 2006-27306A).
Of these proposals, Patent Document 1 describes enhancing steering stability on ice while preventing the sipes cracking or chunk-out failure by disposing two or more sipes in a block face that extend in mutually differing directions so as to cross. Additionally, Japanese Unexamined Patent Application Publication No. 2006-27306A describes enhancing riding comfort while ensuring steering stability and wet braking performance by providing a sipe with a shape having a twist around a twisting axis that extends in a tire radial direction.
However, in the case of Japanese Unexamined Patent Application No. H9-263111A, while the water absorption of the tread surface is enhanced to a certain degree, there is a limitation in that it becomes difficult to maintain steering stability due to a decline in tread rigidity if further enhancements of the water absorption are attempted by increasing the number of sipes. Additionally, in the case of Japanese Unexamined Patent Application Publication No. 2006-27306A, due to a twist angle being set to a size reaching 135° and greater, there are problems such as releasability from a mold after the tire is vulcanization molded being negatively affected and the tread surface becoming easily damageable.
Moreover, studless tires having sipes provided in a tread surface thereof may be used as summer tires after the tire has become worn and finished its role as a winter tire. Generally, rubber is used for the tread rubber of studless tires that is softer than that used for the tread rubber in summer tires. Therefore, there is a problem in that with such a tire, traveling performance as a summer tire cannot be ensured after the tire has finished its role as a winter tire. As a result, the level to which traveling performance as a summer tire is ensured at latter stages of wear after the tire has finished its role as a winter tire is an important issue for such studless tires as those described above.